1. Field of the Invention
The invention relates to microfibers made from ethylene/carboxylic acid ionomers, the fibers being meltspun, particularly using the `meltblown` process. The microfibers in the form of a web material can be efficient gas filters without being electretized. The materials, however, may also be electretized.
2. Description of Related Art
Non-woven materials are well known and widely used in a variety of applications including apparel, adhesives, sorbents and filters. These materials are made from matted, entangled non-bonded, but also melt-bonded fibers. When the matting is very tight, the non-woven material may be thought of as fabric-like. Such non-wovens may be useful, for instance, for apparel. When the matting is relatively loose and open it may considered, and is often referred to as, a `web`. The form of a web may be thick or thin and the fibers entangled and/or bonded with varying degrees of openness, and with variations in the degree of bonding.
The properties of non-woven materials vary widely, from tough to relatively weak, flexible to stiff, highly porous to having low porosity, with highly absorptive or less absorptive capacity, yet may even have barrier characteristics, especially to liquids. The nature of non-wovens depends on (i) what material is used to make the fibers (ii) the nature of the fibers--which will depend on the process used to make them, and (iii) how the fibers are bonded together. It may be fair to say that the variation in the type and applications of non-wovens is as great or greater than that of woven materials.
A major market for non-wovens is in the filtration market, for gases and liquids. Gas filtration involves removing particulate matter, usually solid such as dust, but also liquid particles, from a gas, particularly air. Typical markets include heating ventilating and air conditioning (HVAC). Demanding markets such as pharmaceuticals, microelectronics and biotechnology use highly efficient or ultra efficient particulate air filters (HEPA and UFPA).
Because of the differing gas filter applications, the demands of, and characteristics of gas filters are varied. The filter may require high flux (i.e., high gas throughput) requiring high permeability, and/or require a high level of particle removal, and/or require removal of specific size particles. To a large extent flux, often quantified and measured in terms of the `pressure drop` across the web, and the `filtration efficiency` are conjugate quantities. Thus tighter filter webs may be more efficient but have lower permeability. The filtration efficiency however must always be characterized in relation to the particle size filtered. The drive in filter technology is towards improving the efficiency at given flux, or improving flux at a given efficiency--as well as reducing cost, of course.
The `architecture` of a filter web depends on the fiber diameters, and the distribution of the diameters, how the fibers are entangled/bonded together, the density of web, its uniformity, and the thickness and weight per unit area. These architectural features are a major factor in filter efficiency, the flux, and the size particles which they remove. To filter out small particles, it is necessary to have fine fibers and small passages throughout the web, with no large channels (somewhat comparable to a finer mesh in a woven filter). Webs made of `microfibers` as opposed to fibers of the size of normal textile fibers are used for filtering fine particles in the 0.1 to about 20 micron region.
While fibers for filter webs are often made of glass, synthetic thermoplastic polymers are also commonly used. Microfibers of synthetic thermoplastic polymers are commonly meltspun, particularly by a process known as the `meltblown` process, though certain other processes can produce microfibers such as fibrillating film. The meltblown process has been succinctly defined as `a one-step process in which high-velocity air blows a molten thermoplastic resin from an extruder die tip, onto a conveyor or take-up screen, to form a fine-fibered self-bonding web`. A meltblowing apparatus suitable for the production of microfibers was described in Report No. 4364 of the Naval Research Laboratories, published May 25, 1954, entitled `Manufacture of Super Fine Organic Fibers`, by Van Wente et al.
The ability to make microfibers from synthetic polymers, and the nature of the resulting web produced in the meltblown process depends on the melt rheological and crystallizing or, more generally, solidifying characteristics of the polymer. Other technologies that, in a broader sense, could be considered meltblown, or more generally meltspun processes, and can produce microfibers include electrostatic melt-spinning, flash spinning and centrifugal spinning.
For filtering fine particles however, the appropriate web architecture is a necessary factor but may not be the only factor in determining efficiency. Another major factor is the electrostatic nature of the surface of the polymer fibers. This is significantly dependent on the chemical nature of the polymer composition, the molecular conformation within the fiber, and the surface nature of the fibers made from it. Generally, filter microfibers are subjected to a surface treatment to increase their electrostatic charge or polar nature. So-called `insulating` polymers, those with high resistivity, are `electretized` which is sometimes said to mean `electrified to possess permanent dielectric polarization` or `electrified to make an electret` or to possess an `electret surface`.
A deliberately produced electret surface on filter web microfibers may be produced at different stages of forming the filter. A material may be treated even before fiber formation, such as a sheet before fibrillation. Fibers may be treated during or after their formation, or the treatment may be carried out during or after the actual web formation Such treatment is conventionally done by a procedure involving rubbing or corona charge treatment. Other techniques for providing electret enhancement are described in U.S. Pat. No 4,375,718 (Wadsworth); U.S. Pat. No. 4,588,537 (Klaase) and U.S. Pat. No. 4,592,815 (Nakao).
U.S. Pat. No. 4,215,682 (Kubik et al.) teaches that filtering efficiency of a meltblown microfiber web can be improved by a factor of two or more when made into an electret.
While electrets are often referred to as having a `permanent dielectric polarization`, which leads to or is associated with a surface charge, in fact to a greater or lesser degree, the electrostatic charge or permanence of surface polar nature, whatever its precise nature, decays with time. The usefulness of electret enhanced filtering is of course dependent on how permanent the electret nature is, in relation to the time span for use of the filter.
The most commonly used polymer for making such electret filters is polypropylene but other fibers have been used. U.S. Pat. No. 5,411,576 (Jones et al.) states that other polymers may be used, such as polycarbonates and polyhalocarbons, that may be meltblown and have appropriate volume resistivities under expected environmental conditions.
U.S. Pat. No. 4,626,263 (Inoue) discusses the characteristics of electets made of non-polar and polar polymers. Non-polar polymers such as polyethylene and polypropylene are indicated to produce stable electrets, but of relatively low electrostatic charging capacity, while polar polymers are indicated to have high initial electet capacity, but relatively rapid decay, particularly under humid conditions.
U.S. Pat. No. 4,789,504 (Ohmori et al.) discloses microfibers which are able to be more permanently electretized than prior art materials. The materials used to make the fibers consist of polymers which include polypropylene, polyethylene, polyester, polyamide, poly(vinyl chloride), poly(methyl methacrylate) etc., containing 100 ppm or more, preferably 200 to 2000 ppm, in terms of the metal, of a fatty acid salt such as an aluminum, magnesium or zinc salts of palmitic, stearic or oleic acid.
Electretizing generally requires extra steps and use of special equipment after or during making of the fibers or webs. There is a need for materials which can be made into filter webs without the need to post-charge or deliberately electretize, yet which have comparable or better efficiency/flux. At the same time, if such materials can be electretized to produce filters with even greater efficiency, then the materials serve yet an added need.